Up a steep flight of stairs in a slightly shabby building near London's Victoria station is a GP's surgery. The only clue, among the plastic toys and piles of magazines in the waiting room, that it lies at the heart of the national debate over the safety of the controversial measles, mumps and rubella vaccine is a picture propped up on one of the women GPs' desks. It features a family of grateful patients: Tony, Cherie and Leo Blair.

The surgery is where the Prime Minister's wife brought their son for mother and baby classes. It is also where, if the family do follow Government advice, she should have brought him for that jab.

Today The Observer reveals the strongest indication yet from those close to the Prime Minister that his 19-month-old son, Leo, has, in fact, been given the MMR injection. Downing Street has so far insisted that what transpires behind a GP's office door must remain private on grounds of medical confidentiality. But the question now is whether the Blairs, who have famously kept a rigid grip on their family's privacy, should admit the fact publicly and instil national confidence in a life-saving public health measure.

The debate has shifted from whether the vaccine is safe - as almost the entire medical establishment insists it is - to whether politicians should lead by example. Does the Blairs' right to privacy outweigh the risk of a measles epidemic if parents, convinced the Prime Minister knows something they don't, refuse the vaccine?

Some critics believe Blair's refusal to say anything publicly is hampering efforts to increase MMR uptake. The latest figures show that parents need reassuring. In the first three months of this year, 72 per cent of two-year-olds in London had the MMR vaccination, against the 85 per cent needed to guarantee 'herd immunity' - the point where a sufficient number have had the jab to guarantee the virtual elimination of measles. Uptake is higher elsewhere in the country but often still below the crucial 85 per cent figure.

Dr Ian Gibson, Labour MP and one of the leading medical experts at Westminster, said Blair should set an example by talking about his personal response to the MMR issue. But the Blairs are only the latest prominent family to be drawn into this simmering row. The first personal challenge was in January when the then pregnant Public Health Minister, Yvette Cooper, was handling a routine Commons debate on vaccination. The Tory health spokesman, Philip Hammond, stung by allegations of scaremongering about MMR, retorted that 'all three of my children aged under seven have been vaccinated with the triple dose vaccine'. Cooper was not forthcoming, but when journalists pressed her afterwards, she told them her daughter Meriel 'has had all her vaccinations' as recommended.

Cooper was still on her last days of maternity leave with her second child Joel when the furore over Leo Blair began two weeks ago. As fellow Ministers rejected enquiries, she told her press officers that her views were already public and could thus be repeated.

Then the Daily Mail stoked up the story. On 11 December, the day after a study commissioned by the Government on autism concluded there was no link between the condition and MMR, the paper's parliamentary sketchwriter Quentin Letts revealed that his four-year-old son Claud is autistic. The boy has had the triple vaccine, and Letts said that while he did not know whether that caused his illness, it came as a 'vicious kick in the guts' that Cherie Blair refused to discuss Leo's vaccination. Parents felt 'stupid and duped', worrying that she knew something they did not, he said. And so began a Mail campaign, with all leading MPs known to have young children being asked to declare whether they had inoculated them.

Two events kept the subject in the public eye last week. On Wednesday Julie Kirkbright, ex-journalist turned Tory MP, who has not had her 14-month-old son vaccinated, asked the Prime Minister during Question Time whether he practised what he preached concerning MMR. Blair refused to answer.

Then came the Today interview. Last Tuesday Radio 4 called the Department of Health, asking that a Minister participate in a programme on racism in social work departments. Jacqui Smith, the clean-cut Minister for Health agreed to the request. On Wednesday morning the Today presenter John Humphrys discussed that day's interviews with staff and it was agreed that Smith would also be asked about vaccination. Nobody told her what was coming up - a mistake, senior BBC staff now agree.

Five minutes into the discussion and Humphrys pounced. Smith was caught. She knew Downing Street had ordered Ministers not to talk about their own children. If they did, the question to the Prime Minister on MMR would be far harder to fend off.

Humphrys asked Smith 10 times why she was refusing to reveal the information. The fuchsia-jacketed former teacher stuck to the privacy line, saying the health of her children was not public business. Humphrys accused her of being 'holier than thou'. Smith said she 'fully supported Government policy [on MMR]', a reference later taken to mean that her children had been given the injection.

Later that morning the Department of Health's director of communications, Sian Jarvis, had an angry conversation with Rod Liddle, editor of the programme. Smith had finished the interview 'genuinely distressed', she said. Liddle said the questions were perfectly legitimate. The public was left nonplussed and uncertain.

With his traditional cup of tea to hand, the mood of Alastair Campbell, Blair's director of communications, was clear as he entered the study at No 10 that same morning for his daily strategy meeting with staff. Smith had been 'kebabed', caught off-guard about MMR without being forewarned. Downing Street would now spend the rest of the day fending off calls about whether Leo had been given the MMR jab. They refused to comment.

After the meeting Campbell went back to his office and began writing. The letter, addressed to Richard Sambrook, BBC head of news, Steve Mitchell, head of radio news, and Liddle was excoriating. How did the interview fit in with BBC guidelines about patient confidentiality and rules on not invading children's privacy? Why was the programme obsessed with 'process and personality'? The 'holier than thou' reference had been particularly offensive. And then came the killer comment. There was a time, Campbell said, when the Prime Minister would consider Today his first port of call when sifting the myriad of interview requests. That time was now over.

For No 10, MMR was a 'line in the sand issue'. Give an answer about Leo Blair, and the next question on the children's health would be less easy to block. Sex education? Smoking? Drinking? There was also the matter of patient confidentiality. 'If the Prime Minister started using his children to promote Government policy then you would hear howls of protest,' a No 10 official said. 'What will they want next - a picture of Leo with a needle stuck in his thigh?'

Today has robustly defended the programme, saying that although Smith should have been told she was going to be asked about the vaccination issue it did not need a rocket scientist to work out that she might be, given that it is a responsibility of her department. 'I know the Government would just like to come on and say how well the country is doing, and we say "thank you very much",' said one senior BBC figure. 'But it's not like that now - we are setting an agenda, not just following it.'

The sideshow in the media hides a far more serious problem now facing the Government: how to restore public confidence in the vaccine and tackle the climate of fear that has created a growth industry for private medicine. Some parents have been persuaded to pay up to £200 a shot to take the vaccines singly in private clinics, 10 times the cost price of the vaccine.

When the latest MMR scare hit in January, the weight of calls to the London clinic Direct Health 2000 - the leading supplier of single jabs - caused its computers to crash. It had to hire an army of temporary staff to answer the phones.

The clinic has since taken on extra nursing staff to tackle a waiting list of babies. At £60 per shot there is no shortage of takers: it says it has administered 10,000 doses in the last year, even though it is not allowed to advertise its services.

Critics have said that the Blairs' silence has allowed the media to create a new sense of panic about the issue, and GPs say that the arguments about the safety of MMR are now harder to make.